Taken together, the impacts of
greenhouse gases around the globe should far outweigh
the regional effects of land-cover change, according to
Feddema. However, the regions with extensive agriculture
and deforestation also tend to be highly populated, so
the effects of land-cover change are often focused where
people live.
"Compared to global warming, land use is a relatively
small influence. However, there are regions where it's
really important," he says.
To bracket a range of possibilities, the group
examined two contrasting scenarios for greenhouse
emissions and land cover put forth by the
Intergovernmental Panel on Climate Change. The more
pessimistic scenario assumed that emissions will
increase steadily, while the more optimistic scenario
assumes rapid gains in energy efficiency.
The results for the first scenario show that
deforestation adds 2°C (3.6°F) or more to surface
temperature across the Amazon by 2100. Cooling occurs in
the nearby Pacific and Atlantic waters with a weakening
of the large-scale Hadley circulation that drives
tropical and subtropical climate. In turn, moisture
penetrates further north and produces a cooling,
moistening influence across the U.S. Southwest during
that region's summer monsoon.
While deforestation acts to warm the tropics by
replacing forests with less productive pasture,
converting midlatitude forests and grasses to cropland
tends to act as a cooling influence, because the crops
tend to reflect more sunlight and release more moisture
into the air. Feddema and colleagues found that expanded
agriculture tends to counteract global warming by as
much as 50% across parts of North America, Europe, and
Asia. In Canada and Russia, boreal forests add to
regional warming as they spread north over time.
Although the two IPCC scenarios studied agree on the
impacts of land use in some regions, they produce
contrasting results in others. The next step for Feddema
and colleagues is to utilize the NCAR-based Community
Climate System Model, which will provide
higher-resolution results. They also hope to incorporate
the effects of urban areas on regional climate.
"Our results suggest that more research efforts
should be devoted to producing viable scenarios of
land-cover change in the future," says coauthor Linda
Mearns, director of the NCAR Institute for the Study of
Society and Environment. "We very much hope that other
climate modeling centers perform experiments similar to
ours."
"The purpose of our project is to include human
processes more directly in global climate models," adds
Feddema. "This is the first step."
The simulations were supported by the DOE, the
National Science Foundation, and the University of
Kansas. NSF is NCAR's primary sponsor. Opinions,
findings, conclusions, or recommendations expressed in
this publication do not necessarily reflect the views of
NSF.